The present invention relates to control systems for electric motor powered traction vehicles such as locomotives, transit vehicles or off-highway vehicles and, more particularly, the invention relates to a method for controlling such a vehicle in a manner to correct for wheel slip or slide during propulsion and electrical retarding, respectively.
Locomotives and transit vehicles as well as other large traction vehicles are commonly powered by electric traction motors coupled in driving relationship to one or more axles of the vehicle. Locomotives and transit vehicles generally have at least four axle-wheel sets per vehicle with each axle-wheel set being connected via suitable gearing to the shaft of a separate electric motor commonly referred to as a traction motor. In the motoring mode of operation, the traction motors are supplied with electric current from a controllable source of electric power (e.g., an engine-driven traction alternator) and apply torque to the vehicle wheels which exert tangential force or tractive effort on the surface on which the vehicle is traveling (e.g., the parallel steel rails of a railroad track), thereby propelling the vehicle in a desired direction along the right of way. Alternatively, in an electrical braking mode of operation, the motors serve as axle-driven electrical generators. Torque is applied to their shafts by their respectively associated axle-wheel sets which then exert braking effort on the surface, thereby retarding or slowing the vehicle's progress. In either case, good adhesion between each wheel and the surface is required for efficient operation of the vehicle.
Maximum tractive or braking effort is obtained if each powered wheel of the vehicle is rotating at such an angular velocity that its actual peripheral speed is slightly higher (motoring) or slightly lower (braking) than the true vehicle speed (i.e., the linear speed at which the vehicle is traveling, usually referred to as "ground speed" or "track speed"). The difference between wheel speed and track (or "ground") speed is referred to as "slip speed." There is a relatively low limit value of slip speed at which peak tractive or braking effort is realized. This value, commonly known as maximum "creep speed," is a variable that depends on track speed and rail conditions. So long as the maximum creep speed is not exceeded, slip speed is normal and the vehicle will operate in a stable microslip or creep mode. If wheel-to-rail adhesion tends to be reduced or lost, some or all of the vehicle wheels may slip excessively, i.e., the actual slip speed may be greater than the maximum creep speed. Such a wheel slip condition, which is characterized in the motoring mode by one or more spinning axle-wheel sets and in the braking mode by one or more sliding or skidding axle-wheel sets, can cause accelerated wheel wear, rail damage, high mechanical stresses in the drive components of the propulsion system, and an undesirable decrease of tractive (or braking) effort.
Many different systems are disclosed in the prior art for automatically detecting and recovering from undesirable wheel slip conditions. Typically, differential speeds between axle-wheel sets or rate of change of wheel speed or a combination of these two measurements are used to detect wheel slip. Speed is monitored and if found to exceed predetermined differentials or rates of change, power to the motors is reduced in an attempt to bring speed to a value at which traction is regained. The disadvantage of such systems of wheel slip control is that the controlled variable, i.e., creep speed, is also the signal used to enable correction.